1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of effectively performing a light beam with a pulse width modulation using various gray-scale pulses.
2. Discussion of the Background
Conventionally, electrophotographic is widely applied to an image forming apparatus such as a laser printer using a semiconductor laser. In this apparatus, the laser light beam is deflected with a polygon mirror to scan a surface of a photosensitive member, thereby forming a latent image of an object image.
As an example of this type of apparatus, Japanese unexamined patent application publication No. 08-018782 describes a multiple color information recording apparatus attempting to improve an image quality with a pulse width modulation (PWM) method. The PWM method controls a time period defined as a duty of an on-time of a laser diode.
Also, a technique is known which improves an image quality by increasing a recording resolution. In this technique, a laser diode is driven to emit laser light at a density more than twice an input image density, or a number of input pixels, in at least one of the main scanning and sub-scanning directions. In recent years, this technique is widely used to improve the image quality, in which the recording is performed at such a high resolution with a reduction of a number of gray-scale included in image data.
Generally, a dither method and an error diffusion method are used for a case where a gray-scale is expressed with binary image data, for example. The PWM method can also be used for such case. In the PWM method, a pulse is predefined with a certain width and a dot is formed using this pulse. In order to obtain a superior image quality, it is needed to select a pulse having the most preferable width.
The PWM method can also be used for a case where a gray-scale is expressed with image data of a multiple value such as a four-stepped-value, an eight-stepped-value, and so forth. In the multiple value, the four-stepped-value has three levels of data, the eight-stepped-value has seven levels of data, and so forth. To each of these levels of data included in each multiple value, a pulse is predefined with a width. In this case, it is also needed to select a pulse having the most preferable width so as to obtain a superior image quality.
A free selection of pulses is preferable to obtain a pulse having the most suitable width. However, it is difficult to predefine a pulse width by dividing it into relatively small steps, which increases a manufacturing cost, particularly in a case where a recording is performed at a relatively high resolution and at a relatively fast recording speed.
Therefore, in an apparatus that generates, for example, eight different pulses including one-eighth, two-eighth, three-eighth, four-eighth, five-eighth, six-eighth, seven-eighth, and eight-eighth, a pulse having the most suitable width is required to be selected from among these eight different pulses relative to the data of each gray-scale. As a result, it produces a drawback that an image quality is not optimized.
In recent years, a red laser diode having a 650-nm-ranged wave length that is capable of reducing a spot size has been used as a light source in place of a laser diode of a 780-nm-ranged wave length to be able to perform a recording at a high resolution. However, this red laser diode has a characteristic that, as illustrated in FIG. 11, a start of an actual light emission indicated by a signal S2 is delayed relative to a signal indicated by a signal S1 for commanding the laser diode to start emitting laser light. Accordingly, laser light needed may not be obtained when the width of a pulse is predefined without adjusting the above-mentioned time delay. Consequently, this causes a problem that an image quality is degraded.
Actually, when a recording is performed with a red laser diode, the pulses having widths at relatively low duty ratios are not usable depending upon a recording resolution or a recording speed. This leads to a problem that the narrow selection becomes more narrow.
The above-mentioned Japanese unexamined patent application publication No. 08-018782 describes a multiple color information recording apparatus capable of selecting a method of growing pulses with the pulse width modulation in accordance with input data and values of the neighboring pixels. Such an apparatus can select the positions of pulses but not solve the above-mentioned problem of the narrowed selection.